As investigators around the world chase down leads in the foiled parcel-bomb plot, they have focused on one potential suspect: Ibrahim Hassan al-Asiri, a 28-year-old Saudi explosives expert and likely operative of the Yemen-based al-Qaeda in the Arabian Peninsula (AQAP). Authorities believe Asiri is linked to two previous plots using the same type of PETN explosives found in the printer bombs: the thwarted-in-midair "underwear bomber" attack on a flight from Amsterdam to Detroit on Dec. 25, 2009, and an attempted assassination in Saudi Arabia earlier that year. In August 2009, a man believed to be Asiri's younger brother Abdullah was identified as the suicide bomber who tried to kill Prince Mohammed bin Nayef, Saudi Arabia's Minister of the Interior, who is in charge of internal security for the country. Asiri is alleged to have recruited his brother into al-Qaeda and into the plot. In the recent incident, according to news reports, Asiri designed the bombs to fit inside Hewlett-Packard printers that were packed innocuously along with books, souvenirs and clothing.

Saudi intelligence, reportedly based on the testimony of an al-Qaeda turncoat, may have helped discover the two parcel bombs, which were apparently designed to explode aboard airplanes. But security forces around the globe seem to be playing a game of catch-up when dealing with plots to subvert weaknesses in the global flight-and-cargo network. "Terrorism is an area where you have to keep fighting yesterday's wars even as you look for where the next front will be opened with new kinds of plots," says a European antiterrorism official. "Each new plot and strategy that we prevent forces the terrorists to start over again and figure out some new way to attack us. We learn from each plot we stop, but unfortunately, they do too." (See how al-Qaeda in Yemen is becoming more lethal.)

John O. Brennan, President Obama's top antiterrorism adviser, suggested on CNN that the bombs' construction didn't require someone aboard to "physically detonate them"  raising the likelihood that they were meant to blow up midflight, as British Prime Minister David Cameron suggested over the weekend. Indeed, as the BBC reported, disaster may have been averted only because Jaber al-Faifi, a former Guantánamo inmate who returned to the al-Qaeda fold after his release, repented and turned whistle-blower on the Asiri plot two weeks ago. (See pictures of released Guantánamo detainees.)

Governments are now scrambling to close the gaps that allowed the explosives-laden packages, which originated in Yemen, to go through as unscreened cargo  and use lessons learned from the thwarted plot to anticipate how terrorists may try to innovate in future attempts. Though passengers and luggage are fully screened on regular flights, inspection of cargo differs greatly from country to country. In August 2010, the U.S. began requiring all air cargo carried on domestic passenger flights to be scanned, but it has yet to place such restrictions on freight-only flights. The U.K., meanwhile, uses a "trusted shipper" system under which, after initial audits, a company's cargo is considered safe for transport without regular screening by transport authorities. Procedures in other countries  using X-ray machines, sniffer dogs or manual and visual verification  can run from rigorous to nearly nonexistent, and at times they are left to individual companies to oversee. Still, security officials have told the press that the two parcel bombs discovered last week were so sophisticated in replicating the printers' internal structure that they might well have slipped through even close screening. (See how al-Qaeda operates in Yemen.)

"Friday's incident shows that al-Qaeda is well aware of this loophole in the system, and they fully intend to exploit it," said Massachusetts Representative Edward J. Markey, author of the law requiring all cargo loaded on passenger flights to be checked, in a statement promising similar legislation for all cargo. "It is time for the shipping industry and the business community to accept the reality that more needs to be done to secure cargo planes so that they cannot be turned into a delivery system for bombs targeting our country."

The U.K. banned freight deliveries from Yemen on Oct. 28 as concerns mounted that the two explosives-packed parcels discovered might have been among several yet undetected. France and Germany followed suit the day after. On Monday, Yemen announced it was imposing "exceptional security measures on all cargo leaving Yemeni airports."